Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"POLYOLEFIN COMPOSITION HAVING A HIGH BALANCE OF STIFFNESS
AND IMPACT STRENGTH"
The present invention relates to an elastomeric thermoplastic polyolefin
composition.
In particular, the present invention relates to compositions containing a
broad molecular
weight distribution propylene polymer.
Due to its mechanical and physical properties, the polymer composition of the
present
invention finds application above all in automotive field (e.g. bumpers and
side strips).
Such a polyolefin composition has a good balance of mechanical properties, in
particular improved balance of flexural modulus and IZOD impact strength even
at low
temperatures (e.g. at -30° C).
An added advantage, which is shown by the composition of the present
invention, is
that it presents low values of thermal shrinkage. Said property imparts a
higher dimensional
stability to the articles produced with the polyolefin composition of the
present invention.
In WO00/26295 polyolefin compositions with low values of coefficient of linear
thermal expansion and good mechanical properties are described, comprising
(by. weight)
from 40 to 60% of a broad molecular weight distribution propylene polymer
having a
polydispersity index from 5 to 15 and melt flow rate of from 80 to 200 g/10
min (according
to ASTM-D 1238, condition L); and from 40 to 60%, of a partially xylene-
soluble olefin
polymer rubber containing at least 65% by weight of ethylene, the IVs/IVA
ratio between the
intrinsic viscosity (IVs) of the portion soluble in xylene of the polyolefin
composition at
room temperature and the intrinsic viscosity (IVA) of the said propylene
polymer ranging
from 2 to 2.5.
These compositions typically have a flexural modulus of from 650 to 1000 MPa.
However, for certain automotive applications it desirable to have compositions
with
flexural modulus values of higher than 1000 MPa, in particular higher than
1100 MPa, still
maintaining a good balance of overall mechanical properties and low values of
thermal
shrinkage.
It has been surprisingly found that such balance of properties can be achieved
by a
combining, in compositions containing a propylene polymer and an olefin
rubber, a low
content of the said rubber with a particular selection of features of the
propylene polymer.
Therefore an object of the present invention is a polyolefin composition
comprising
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(percentage by weight):
(A) from 60 to 85%, preferably 60 to 80%, of a broad molecular weight
distribution
propylene polymer (component A) having a polydispersity index from 5 to 15 and
melt flow rate of from 20 to 78 g/10 min, preferably from 40 to 75, more
preferably
from 40 to 70 g/10 min (according to ASTM-D 1238, condition L); and
(B) from 15 to 40%, preferably 20 to 40%, of a partially xylene-soluble olefin
polymer
rubber (component B) containing at least 65% by weight of ethylene.
The method for measuring the xylene-soluble content and polydispersity index
are
described hereinbelow. The room temperature means a temperature of about
25° C in the
present application.
The polyolefin composition of the present invention may further contain a
mineral
filler. When present, it is contained in an amount from about 0.5 to 3 parts
by weight with
respect to the sum of components (A) and (B).
The composition of the present invention typically has a melt flow rate of
from 5 to
20 g/10 min. The intrinsic viscosity of the fraction soluble in xylene at room
temperature
(about 25 °C) of the overall composition is preferably of from 2 to 2.7
dl/g.
In addition, typically, it has a flexural modulus of from 1100 to 1700 MPa.
Preferably
the value of thermal shrinkage is from 0.5 to 1 in the longitudinal direction,
and from 0.7 to
1.2 in the transversal direction;the notched IZOD resilience at -30° C
is typically from 4 to
KJ/m2. The methods for measuring the said properties are described
hereinbelow.
Component (A) is a crystalline propylene homopolymer or a propylene copolymer
with ethylene or C4-Clo a-olefin or a mixture thereof. Ethylene is the
preferred comonomer.
The comonomer content ranges preferably from 0.5 to 1.5% by weight, more
preferably from
0.5 to 1 % by weight.
A xylene-insoluble content at 25°C of component (A) is typically
greater than 90%,
preferably equal to or greater than 94%.
Component (A) preferably has a molecular weight distribution MW/M", (MW =
weight
average molecular weight and M" = number average, molecular weight, both
measured by gel
permeation chromatography) of from 8 to 30.
Preferably, component (A) comprises from 30 to 70% by weight, more preferably
from 40 to 60% by weight, based on the total weight of (A), of a fraction (AI)
having melt
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flow rate of from 1 to 10 g/10 min..
The olefin polymer rubber of component (B) used in the polyolefin composition
of
the present invention can be a polyethylene-co-C3-Clo a-olefin) or
polyethylene-co-
propylene-co-C4-Clo a-olefin) having an ethylene content preferably from 65 to
80 % by
weight. The latter contains about from 0.5 to 10% by weight of a C4-Clo a-
olefin. The olefin
polymer rubber can optionally further contain a dime, the content of which is
preferably of
from 1 to 10% by weight, more preferably from 1 to 5% by weight.
The olefin polymer rubber of component (B) is partially soluble in xylene at
room
temperature. The xylene-insoluble content is about 25-40% by weight,
preferably 30-38% by
weight.
The C3-C1o a-olefins useful in the preparation of component (B) described
above
include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-
octene.
Propylene and 1-butene are particularly preferred.
The mineral filler, when present, is preferably selected from talc, calcium
carbonate,
silica, conventional clays, wollastonite, diatomaceous earth,' titanium oxide
and zeolites.
Preferably, the mineral filler is talc.
In addition to the mineral fillers discussed above, the polyolefin composition
of the
present invention may further contain conventional additives, for example,
stabilizers,
pigments, other fillers and reinforcing agents, e.g. carbon black and glass
beads and fibers.
The polyolefin composition of the present invention can be prepared by way of
a
physical blend or chemical blend.
Preferably, the composition of the present invention is prepared directly in
polymerization by sequential polymerization processes in a series of reactors
based on the
use of particular stereospecifc Ziegler-Natta catalysts, producing by
polymerization a mixture
of component (A) and component (B). Subsequently, the mineral filler is,
optionally, added
by blending, or in the final pelletization section of the industrial
polymerisation plant.
The polymerization process is carried out in at least three consecutive
stages, in the
presence of particular stereopecific Ziegler-Natta catalysts, supported on a
magnesium halide
in active form. In particular, the broad molecular weight distribution
propylene polymer of
component (A) described above can be prepared by sequential polymerization in
at least two
stages and the olefin polymer rubber in the other stage(s).
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Alternatively, the polyolefin composition of the present invention can be
physically
blended or admixed in any conventional mixing apparatus, such as an extruder
or a Banbury
mixer, by mixing components (A) and (B) and optionally further components.
Components
(B) and (A) are blended in the molten or softened state. Thus components (A)
and (B) can be
prepared in separate polymerization stages, preferably with the Ziegler-Natta
catalysts and
the polymerization conditions hereinafter described, and then mechanically
mixed.
As previously mentioned, the polymerization stage can be carried out in at
least three
sequential steps, wherein components (A) and (B) are prepared in separate
subsequent steps,
operating in each step in the presence of the polymer formed and the catalyst
used in the
immediately preceding step. The catalyst is added only in the first step,
however its activity
is such that it is still active for all the subsequent steps..The order in
which components (A)
and (B) are prepared is not critical. However, it is preferred to produce
component (B) after
producing component (A).
The catalyst used for preparing component (A) is preferably characterized in
that it is
capable of producing propylene polymers having a xylene insoluble fraction at
25° C greater
than or equal to 90% by weight, preferably greater than or equal to 94%.
Moreover, it has a
sensitivity to molecular weight regulators high enough to produce propylene
homopolymers
having a melt flow rate in the range from 1 to 20 g/10 min and greater than
200 g/10 min.
Methods of preparing the broad molecular weight distribution propylene polymer
of
component (A) of the present invention are described in the European patent
application 573
862.
The above said catalyst is used in all the steps of the polymerization process
of the
present invention for producing directly the sum of components (A) and (B).
Catalysts having the above mentioned characteristics are well known in the
patent
literature; particularly advantageous are the catalysts described in US patent
4,399,054 and
European patents 45977 and 395083.
The polymerization process can be carried out in continuous or in batch,
according to
known techniques and operating in liquid phase, in the presence or absence of
inert diluent,
or in gas phase or in mixed liquid-gas phases. It is preferable to operate in
gas phase.
Reaction time and temperature are not critical; however, the temperature
typically
ranges from 20 to 100° C.
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Preferably, the reaction temperature is generally from 60 to 85° C
for the
polymerization of component (B).
Regulation of the molecular weight is carried out by using known regulators
such as
hydrogen.
The compositions of the present invention can also be produced by a gas-phase
polymerisation process carried out in at least two interconnected
polymerisation zones. The
said type of process is illustrated in European patent application 782 587.
In detail, the above-mentioned process comprises feeding one or more monomers)
to said
polymerisation zones in the presence of catalyst under reaction conditions and
collecting the
polymer product from the said polymerisation zones. In the said process the
growing
polymer particles flow upward through one (first) of the said polymerisation
zones (riser)
under fast fluidisation conditions, leave the said riser and enter another
(second)
polymerisation zone (downcomer) through which they flow downward in a
densified form
under the action of gravity, leave the said downcomer and are reintroduced
into the riser,
a thus establishing a circulation of polymer between the riser and
the.downcomer.
In the downcomer high values of density of the solid are reached, which
approach the bulk
density of the polymer. A positive gain in pressure can thus be obtained along
the direction
of flow, so that it become to possible to reintroduce the polymer into the
riser without the
help of special mechanical means. In this way, a "loop" circulation is set up,
which is def ned
by the balance of pressures between the two polymerisation zones and by the
head loss
introduced into the system.
Generally, the condition of fast fluidization in the riser is established by
feeding a gas
mixture comprising the relevant monomers to the said riser. It is preferable
that the feeding
of the gas mixture is effected below the point of reintroduction of the
polymer into the said
riser by the use, where appropriate, of gas distributor means. The velocity of
transport gas
into the riser is higher than the transport velocity under the operating
conditions, preferably
from 2 to 15 m/s.
Generally, the polymer and the gaseous mixture leaving the riser are conveyed
to a
solid/gas separation zone. The solid/gas separation can be effected by using
conventional
separation means. From the separation zone, the polymer enters the downcomer.
The gaseous
mixture leaving the separation zone is compressed, cooled and transferred, if
appropriate
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with the addition of make-up monomers and/or molecular weight regulators, to
the riser. The
transfer can be effected by means of a recycle line for the gaseous mixture.
The control of the polymer circulating between the two polymerisation zones
can be
effected by metering the amount of polymer leaving the downcomer using means
suitable for
controlling the flow of solids, such as mechanical valves.
The operating parameters, such as the temperature, are those that are usual in
gas-phase
olefin polymerisation process, for example between 50 to 120 °C.
This process can be carried out under operating pressures of between 0.5 and
10 MPa,
preferably between 1.5 to 6 MPa.
Advantageously, one or more inert gases are maintained in the polymerisation
zones, in
such quantities that the sum of the partial pressure of the inert gases is
preferably between 5
and 80% of the total pressure of the gases. The inert gas can be nitrogen or
propane, for
example.
The various catalysts are fed up to the riser at any point of the said riser.
However,
they can also be fed at any point. of the downcomer. The.catalyst can be in
any physical state,
therefore catalysts in either solid or liquid state can be used.
An essential component of the Ziegler-Natta catalysts used in the
polymerization
process of the present invention is a solid catalyst component comprising a
titanium
compound having at least one titanium-halogen bond, and an electron-donor
compound; both
supported on a magnesium halide in active form.
Another essential component (co-catalyst) is an organoaluminum compound, such
as
an aluminum alkyl compound. An external donor is optionally added.
The solid catalyst components used in said catalysts comprise, as electron-
donors
(internal donors), compounds selected from the group consisting of ethers,
ketones, lactones,
compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic
acids.
Particularly suitable electron-donor compounds are phthalic acid esters, such
as diisobutyl,
dioctyl, diphenyl and benzylbutyl phthalate.
Other electron-donors particularly suitable are 1,3-diethers of formula:
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RI' ~CH20R~
/C
R~ ~ \ CH20R~'
wherein RI and Ra are the same or different and are C1-C1g alkyl, C3-C18
cycloalkyl or C7-C1$
aryl radicals; R~ and R~' are the same or different and are C1-C4 alkyl
radicals; or are the
1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or
polycyclic structure
made up of 5, 6 or 7 caxbon atoms and containing two or three unsaturations.
Ethers of this type are described in published European patent applications
361493
and 728769.
Representative examples of said dieters are as follows: 2-methyl-2-isopropyl-
1,3-
dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2-isopropyl-2-
cyclopentyl-1,3-
dimethoxypropane, 2-isopropyl-2-isoamyl-1,3-dimethoxypropane and 9,9-bis
(methoxymethyl) fluorene.
The preparation of the above mentioned catalyst components is carried out
according
to various methods. For example, a MgCl2~nROH adduct (in particular in the
form of
spherical particles) wherein n is generally from 1 to 3 and ROH is ethanol,
butanol or
isobutanol, is reacted with an excess of TiCl4 containing the electron-donor
compound. The
reaction temperature is generally from 80 to 120 °C. The solid is then
isolated and reacted
once more with TiCl4, in the presence or absence of the electron-donor
compound, after
which it is separated and washed with aliquots of a hydrocarbon until all
chlorine ions have
disappeared. In the solid catalyst component the titanium compound, expressed
as Ti, is
generally present in an amount from 0.5 to 10% by weight. The quantity of
electron-donor
compound which remains fixed on the solid catalyst component generally is 5 to
20% by
moles with respect to the magnesium dihalide. The titanium compounds, which
can be used
for the preparation of the solid catalyst component, are the halides and the
halogen
alcoholates of titanium. Titanium tetrachloride is the preferred compound.
The reactions described above result in the formation of a magnesium halide in
active
form. Other reactions are known in the literature, which cause the formation
of magnesium
halide in active form starting from magnesium compounds other than halides,
such as
magnesium carboxylates.
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The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as
Al-
triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl
compounds containing
two or more A1 atoms bonded to each other by way of O or N atoms, or S04 or
S03 groups.
The Al-alkyl compound is generally used in such a quantity that the Al/Ti
ratio be
from 1 to 1000.
The electron-donor compounds that can be used as external donors include
aromatic
acid esters such as alkyl benzoates and in particular silicon compounds
containing at least
one Si-OR bond, where R is a hydrocarbon radical. Useful examples of silicon
compounds
are (tert-butyl)2 Si (OCH3)2, (cyclohexyl) (methyl) Si (OCH3)a, (phenyl)Z Si
(OCH3)2 and
(cyclopentyl)2 Si (OCH3)2.
1,3-diethers having the formulae described above can also be used
advantageously.
If the internal donor is one of these diethers, the external donors can be
omitted.
The catalysts can be precontacted with small quantities of olefins
(prepolymerization), thus improving both the performance of the catalysts and
the
morphology of the polymers. Prepolymerization .is carried out maintaining the
catalysts in
suspension in a hydrocarbon solvent (hexane or heptane, for example) and
polymerizing at a
temperature from ambient to 60° C for a time sufficient to produce
quantities of polymer
from 0.5 to 3 times the weight of the solid catalyst component. It can also be
carried out in
liquid propylene, at the temperature conditions indicated above, producing
quantities of
polymer that can reach up to 1000 g per g of catalyst component.
As mentioned above, the polyolefin composition of the present invention can
also be
obtained by blending. The blending is done using known techniques starting
from pellets or
powders or particles of the polymers obtained from the polymerization process,
which are
preferably pre-mixed with the mineral filler in the solid state (with a
Banbury, Henshel or
Lodige mixer, for example) and then extruded.
As above-mentioned, the polymer composition of the present invention is
suitable to
prepare bumpers and other parts of vehicles, such as side strips. Hence, the
polymer
composition is subjected to the conventional techniques used to prepare the
said articles.
The following analytical methods are used to characterize the propylene
polymer of
component (A), rubber copolymer of component (B) and the composition obtained
therefrom.
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Melt Flow Rate: determined according to ASTM-D 1238, condition L.
fr~l intrinsic viscosity: determined in tetrahydronaphtalene at 135° C.
Ethylene: determined according to LR. Spectroscopy.
Soluble and insoluble in x l~ 2.5 g of polymer are dissolved in 250 ml of
xylene at 135°
C under agitation. After 20 minutes the solution is allowed to cool to
25° C, still under
agitation, and then allowed to settle for 30 minutes. The precipitate is
filtered with filter
paper, the solution evaporated in nitrogen flow, and the residue dried under
vacuum at 80°C
until constant weight is reached. Thus one calculates the percent by weight of
polymer
soluble and insoluble in xylene at ambient temperature (25° C).
Polydispersit~Index (P.L): measurement of the molecular weight distribution in
the polymer.
To determine the P.I. value; the modulus separation at low modulus value,
e.g., 500 Pa, is
determined at a temperature of 200° C by using a RMS-800 parallel-
plates rheometer model
marketed by Rheometrics (USA), operating at an oscillation frequency which
increases from
0.01 rad/second to 100 rad/second. From the modulus separation value, the P.I.
value can be
derived using the following equation: ,.
P.I. = 54.6(modulus separation)-1~~6
wherein the modulus separation (MS) is defined as:
MS = (frequency at G' = 500 Pa)/(frequency at G" = 500 Pa)
wherein G' is the storage modulus and G" is the low modulus.
Flexural Modulus: determined according to ASTM-D 790.
Tensile streQ-th at yield: ISO method 527.
Tensile strength at break: ISO method 527.
Elongation at break and at yield: ISO method 527.
Lon~,itudinal and transversal thermal shrinkage
A plaque of 100 x 200 x 2.5 mm is moulded in an injection moulding machine
"SANDRETTO serie 7 190" (where 190 stands for 190 tons of clamping force).
The injection conditions are:
melt temperature = 250°C;
mould temperature = 40°C;
injection time = 8 seconds;
holding time = 22 seconds;
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screw diameter = 55 mm.
The plaque is measured 48 hours after moulding, through callipers, and the
shrinkage is
given by:
Longitudinal shrinkage = 200 - ~ ~ o- value x 100
Transversal shrinkage = 100 - r i o _ value x 100
wherein 200 is the length (in mm) of the plaque along the flow direction,
measured
immediately after moulding;
100 is the length (in mm) of the plaque crosswise the flow direction, measured
immediately
after moulding;
the y~ead value is the plaque length in the relevant direction, measured after
48 hours.
Notched IZOD impact test at -30° C: determined according to ASTM-D
256/A.
The following examples are given in order to illustrate and not limit the
present
invention.
Examples, l and 2
Preparation of the solid catal s~ponent
A MgCl2/alcohol adducts in spherical form is prepared following the method
described in example 2 of USP No. 2,399,054 but operating at 3,000 RPM instead
of 10,000
RPM.
The adduct is partially dealcoholated by heating at increasing temperatures
from 30 to
180° C operating in nitrogen current.
In a 1 liter flask equipped with a condenser and mechanical agitator is
introduced,
under a nitrogen current, 625 ml of TiCl4. At 0°C while agitating are
added 25 g of partially
dealcoholated adduct. It is then heated up to 100° C in 1 hour; when
the temperature reaches
40° C diisobutylphthalate (DIBF) is added in molar ratio Mg/DIBF=8.
The temperature is maintained at 100° C for 2 hours. It is then left to
decant and
afterwards the hot liquid is siphoned off. 550 ml of TiCl4 is added and it is
heated to 120° C
for 1 hour. Finally, it is left to settle and the liquid is siphoned off while
hot; the residual
solid is washed 6 times with 200 ml aliquot of anhydrous hexane at 60°C
and 3 times at
room temperature. The solid is then dried under vacuum.
Polymerization
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The polymerization is carried out in continuous in a series of reactors
equipped with
devices to transfer the product from one reactor to the one immediately next
to it.
In the gas phase, hydrogen propane and monomers are continuously analyzed and
fed
in order to maintain constant the desired concentrations.
In the polymerization run a mixture of a triethylaluminum (TEAL) activator and
dicyclopentyldimethoxysilane (DCPMS) as electron-donor component is contacted
with the
solid catalyst component, in such a way that the TEAL/Cat weight ratio is
about 5, in a
reactor at 30° C for about 9 minutes. The TEAL and electron-donor
compound are in such
quantities that TEAL/DCPMS weight ratio is 5.
The catalyst is then transferred to a reactor containing an excess of liquid
propylene
and prepolymerized for 33 minutes at 25° C.
The prepolymer is then transferred to the first reactor in gas phase where the
homopolymerization of the propylene occurs to obtain propylene homopolymers
with low
MFR. The product thus obtained is then transferred into the second reactor,
where propylene
is homopolymerized to obtain .homopolymers with high MFR. Finally, the :
product of the
second reactor is transferred to the third reactor, where ethylene is
copolymerized with
propylene to obtain component (B).
The polymerization conditions used in each reactor are shown in Table I and
the
properties of the products thus obtained are shown in Table II.
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Table I
EXAMPLE 1 2
1 REACTOR
Temperature ( C) 85 85
Polypropylene (wt%) 33 31.5
MFR L g/10 min 3.3 3.4
2 REACTOR
Temperature ( C) 85 85
Polypropylene (wt%) 33 32
MFR L (g/10 min) 57.5 50.6
Xylene soluble (wt%) ~.8 2.5
IVA(dl/g) 1.00 1.00
P.I. 10.1 9.5
3 REACTOR
Temperature ( C) 75 75
Ethylene/propylene rubber34 36.5
(wt%)
C2/(C2+C3) mol 0.481 0.503
Notes: C2 = ethylene; C3 = propylene
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Table II
EXAMPLE 1 2
MFR L (g/10 min) 16.5 12.5
Xylene soluble (wt%) 24.8 26
Ethylene content (wt%) 23 25.6
IVS (dl/g) 2.34 2.46
Flexural modulus (MPa) 1264 1202
Tensile strength at yield22 20
(MPa)
Elongation at yield (%a)7 9
Tensile strength at break14 15
(MPa)
Elongation at break (%a)72 278
~
IZOD resilience at 23 12.7 32
C (KJ/m2)
IZ~D resilience at -30 6.9 7.9
C (KJ/m2)
Longitudinal shrinkage 0.73 0.90
(%)
Transversal shrinkage 0.87 1.06
(%)
Note: IVS = Intrinsic
Viscosity of xylene
soluble fraction.
13
